Question

what is a substrate?
○ a molecule that acts as a catalyst
○ a place on the enzyme where the reaction occurs
○ a place on the product where the enzyme fits
○ a reactant that is catalyzed by an enzyme

Explanation:

• Option A: A molecule that acts as a catalyst is an enzyme (or a catalyst), not a substrate. So A is incorrect.
• Option B: The place on the enzyme where the reaction occurs is the active site, not the substrate. So B is incorrect.
• Option C: The substrate fits on the enzyme's active site, not on the product. Products are the result of the reaction, and the enzyme does not fit on the product in this context. So C is incorrect.
• Option D: A substrate is a reactant that binds to the enzyme's active site and is then catalyzed (undergoes a reaction) by the enzyme to form products. So D correctly defines a substrate.

Answer:

D. a reactant that is catalyzed by an enzyme